The MultiBucketsAggregation.Bucket interface extends Writeable, forcing
all implementation classes to implement writeTo(). This commit removes
the Writeable from the interface and move it down to the InternalBucket
implementation.

* master: (61 commits)
  Build: Move plugin cli and tests to distribution tool (elastic#24220)
  Peer Recovery: remove maxUnsafeAutoIdTimestamp hand off (elastic#24243)
  Adds version 5.3.2 and backwards compatibility indices for 5.3.1
  Add utility method to parse named XContent objects with typed prefix (elastic#24240)
  MultiBucketsAggregation.Bucket should not extend Writeable (elastic#24216)
  Don't expose cleaned-up tasks as pending in PrioritizedEsThreadPoolExecutor (elastic#24237)
  Adds declareNamedObjects methods to ConstructingObjectParser (elastic#24219)
  ESIntegTestCase.indexRandom should not introduce types. (elastic#24202)
  Tests: Extend InternalStatsTests (elastic#24212)
  IndicesQueryCache should delegate the scorerSupplier method. (elastic#24209)
  Speed up parsing of large `terms` queries. (elastic#24210)
  [TEST] make sure that the random query_string query generator defines a default_field or a list of fields
  token_count type : add an option to count tokens (fix elastic#23227) (elastic#24175)
  Query string default field (elastic#24214)
  Make Aggregations an abstract class rather than an interface (elastic#24184)
  [TEST] ensure expected sequence no and version are set when index/delete engine operation has a document failure
  Extract batch executor out of cluster service (elastic#24102)
  Add 5.3.1 to bwc versions
  Added "release-state" support to plugin docs
  Added examples to cross cluster search of using cluster settings
  ...

…4216)

The MultiBucketsAggregation.Bucket interface extends Writeable, forcing
all implementation classes to implement writeTo(). This commit removes
the Writeable from the interface and move it down to the InternalBucket
implementation.